Applications requiring the location of wireless mobile devices are referred to as location-based services (LBS). These services require the identity of the device and the device's location. Wireless infrastructure, such as Wi-Fi access points, can be used to determine the location of Wi-Fi devices based on radio waves received or emitted by the device. Three or more wireless receivers record the received signal strength, time-of-arrival, or the angle-of-arrival of the radio frequency signals from the mobile device. These receivers could be Wi-Fi, Bluetooth, RFID, or other wireless devices. A location server processes the data from these receivers to determine the mobile device's location. When the application needs a device's location, it sends a request to the location server with the device's network identifier. Finally, the location server responds to the application with the device's location.
Wi-Fi infrastructure is widely deployed and the ability to use this existing infrastructure for LBS is desirable. Many LBS require position accuracy of less than one meter to be effective. However, this accuracy is difficult to achieve using wireless location systems due to multipath, especially indoors.
Many wireless systems and techniques exist to determine the position of a radio-frequency device including GPS, Wi-Fi, and ultra-wideband. Various radio wave features can be used to determine location, including receive signal strength (RSS), time of arrival (TOA), angle of arrival (AOA), or time difference of arrival (TDOA) among other techniques. RSS and TDOA are the two most popular Wi-Fi location methods. In a localization system using receive signal strength (RSS), the RSS is measured at each access point (AP) and the distance from the AP to the wireless device is calculated based on a path loss model. Given three or more APs simultaneously measuring the RSS from a wireless device, the wireless device's location can be calculated using multilateration algorithms. The time-difference-of-arrival (TDOA) method allows the distance between the AP and the wireless device to be measured directly. The time it takes for the radio signal to travel from the wireless device to the AP is measured.
Both RSS and TDOA localization methods suffer degraded accuracy from multipath. Multipath is a phenomena where an electromagnetic wave follows multiple paths to a receiver. Multipath is caused by three effects: reflections, scattering, and diffraction. Reflections occur when an electromagnetic wave encounter an obstacle larger in size than the wavelength of the signal. Scattering occurs when an electromagnetic wave encounters an obstacle whose size is smaller than the wavelength of the signal. Diffraction occurs when an electromagnetic wave encounters a surface with irregular edges, and travels along a path other than the line of sight. The wavelength of 2.4 GHz Wi-Fi signals is 12.5 cm. Multipath makes it very difficult to determine locations accurately and degrades both methods of localization. Using RSS for localization, it is difficult to create an accurate propagation model due to multipath. In a TDOA system, it is difficult to find the first arrival due to constructive and destructive multipath arriving shortly after the direct path. For both of these systems, it is difficult to attain better than ten meters of accuracy.
To improve accuracy, pattern matching can be used. For example, an area where location-based services are to be provided can be calibrated during an offline site survey process. During calibration, access point parameters such as RSS, TDOA, or multipath signatures can be recorded throughout the entire space where location-based services are needed. Although the pattern matching localization technique achieves better than two-meter accuracy after a site calibration process, the accuracy can quickly degrade to greater than ten meters.